Effects of isobutyrate and 2-methylbutyrate at different levels of rumen degraded protein on ruminal nutrient digestibility, microbial protein synthesis, and milk production in lactating cows

Effects of isobutyrate and 2-methylbutyrate (IB-2MB) supplementation and its interaction with 2 levels of dietary RDP were investigated in a replicated 4 × 4 Latin square design with 2 × 2 factorial arrangement: 9% or 11% RDP (% of dietary DM) with or without IB-2MB supplementation (0.09% IB and 0.06% 2MB in dietary DM). The level of dietary RDP was mainly adjusted with soybean meal products, and the concentration of RUP was constant (6% of dietary DM) based on National Academies of Sciences, Engineering, and Medicine 2021 requirements. Eight ruminally cannulated Holstein cows in the second lactation (192 DIM ± 41.3 SD; milk yield, 42.6 kg/d ± 4.67 SD) were fed the treatment diets for 4 wk in each period. The triple marker system and reticular sampling technique were used to determine ruminal digestibility of nutrients and microbial protein synthesis. Total collection of feces and urine were collected for 3 d in the last week of each period. Data were analyzed using the PROC MIXED of SAS with treatment and period as fixed effects, and square and cow within square as random effects. The model for rumen fermentation data included time (repeated measure) and its interaction with RDP and IB-2MB as fixed effects (0, 3, 6 h after feeding). Milk yield and DMI were not affected by treatments. The high RDP treatments increased ruminal ammonia, plasma urea concentration, and increased urinary N and urea excretion compared with low RDP. Ruminal digestibility of NDF was lower (23.4% vs. 27.3%) for high RDP compared with low RDP. Milk fat yield was increased for IB-2MB when high RDP was supplied (1.20 kg/d vs. 1.43 kg/d) and was not affected under low RDP. The IB-2MB supplementation increased ruminal concentration of IB and did not affect 2MB concentration. An interaction between RDP and IB-2MB supplementation was observed in which milk fat yield and ECM were greater for IB-2MB supplementation under high RDP, whereas no difference was detected under low RDP. Similar interactions were observed for the concentrations of 18:1 trans-10 and UFA in milk fatty acids, which was decreased for IB-2MB supplementation under high RDP. The IB-2MB supplementation decreased ruminal true digestibility of nonammonia N and ruminal apparent digestibility of NDF and tended to decrease ruminal true digestibility of DM, and OM, although the efficiency of microbial protein synthesis (nonammonia bacterial N per unit of OM digested; efficiency of microbial protein synthesis [EMPS]) increased for IB-2MB supplementation. No effect of RDP levels, IB-2MB supplementation, or their interaction on total-tract DM, OM, NDF, N, and starch digestibility was observed. In conclusion, the IB-2MB supplementation increased milk fat yield only when RDP was sufficient and was likely due to changes in ruminal fatty acid metabolism. Although EMPS was increased, IB-2MB supplementation decreased ruminal apparent NDF digestibility in this experimental condition without affecting total-tract digestibility of NDF.